click plus in the image sources section, and then search for option called "capture sound of the exact application" or sth like that, then find the discord on list and click it, it will add a sound zipper for discord in the sounds mixer section (section in the middle), then you need to just mute discord by moving zipper to the left or by clicking red speaker)
baku69
i know ss is in polish but maybe you will like that now where to search